Producing Low Tar Gases in a Multi-Stage Gasifier

ABSTRACT

A system for gasifying solid matter uses multiple stages to produce low-tar combustible gas includes a first reactor having a fluidized bed to produce hydrogen containing gas, pyrolysis vapors, tars, and char particles at temperature less than the exit of the second reactor and a higher temperature partial oxidation combustor zones. A second reactor includes a higher temperature partial oxidation zone to activate hydrogen and cause cracking of aromatic ring compounds, a co-flow moving granular bed with a char gasification stage to catalyze tar reduction, and control char residence time, and a media screen comprising a parallel wire screen substantially vertically oriented supporting granular media.

This application claims the priority of provisional Application Ser. No.61/397,765. The present invention relates, in general, to gasifyingmaterials such as biomass and waste to produce high quality gas.

FIELD OF THE INVENTION Background

“Tars are the Achilles heel of gasifiers, and many gasifier projectshave failed because of insufficient attention to low tar production orefficient tar destruction”—Tom B. Reed (T. Milne 1998). The highlygeneric term “tar” was uniformly defined in 1998 (at the EU/IA/DOEconference in Brussels) as all organic contaminants of gasification thathave a molecular weight larger than benzene. Several review articleshave been published discussing the nature, formation and destruction oftar from biomass gasification. (Li 2009) (Han 2008) (T. Milne 1998).

A maturation process has been proposed for tar with temperature,progressing from mixed oxygenates (400° C.), to phenolic ethers (500°C.), then alkyl phenolics (600° C.), then heterocyclic ethers (700° C.),then polycyclic aromatics (800° C.), and then larger Polynucleararomatic hydrocarbons (PAH), soot, and coke (900° C.). Elliot, D. C.“Relation of reaction time and temperature to chemical composition ofpyrolysis oils.” Proceedings of the ACS Symposium Series 376, PyrolysisOils from Biomass. American Chemical Society, 1988. Polymerization andsubsequent agglomeration of high molecular weight PAH is described as ahomogeneous pathway to “soot” formation. Homann, K. H., Wagner, H. G.,“Some new aspects of the mechanisms of carbon formation in premixedflames.” Eleventh International Symposium on Combustion. Pittsburgh: TheCombustion Institute, 1967.

Several different classifications of tar have been established. Theseclassifications are related to temperatures. Classification has beendeveloped as follows: “primary tars” are vapors produced at lowertemperatures and are the first evolved in thermal depolymerization ofcellulose, hemicellulose, and lignin—these are mainly oxygenatedcompounds. Next, the secondary and tertiary reaction products of primarytars are termed “secondary tar” and “tertiary tar”. Tertiary tars weresub-classified as tertiary-alkyl and tertiary-polynuclear aromatichydrocarbons (PAH). It is hypothesized that once tertiary tars areformed these may require even higher temperatures and additionalresidence time for thermal destruction.

There are several approaches to achieve adequate reduction of tar afteran initial stage of gasification including thermal cracking, partialoxidation, and catalytic cracking using mineral catalysts or reformingwith metal catalysts. One such method is indirect heat thermal cracking.This method has been discussed in the open literature to reduce tars inraw product gas. In the absence of char, a temperature of 900° C. isinsufficient to achieve much tar destruction. Specifically, a slipstream was filtered at 450° C. to remove all char dust, but nomeasurable difference was found (after a fluid bed gasifier (8000 mg/Nm³in feed gas from CFB operating at 825° C.) was filtered at 450° C. toremove all char dust). The application of a homogeneous phase reactordemonstrated only ˜25% reduction even with residence times as high as 12seconds. Even at 1000° C. with 12 second residence time, only 75%reduction was achieved (˜2000 mg/Nm³ in product). (Houben, M. P.Analysis of tar removal in a partial oxidation burner. PhD Dissertation,Eindhoven: Technical University Eindhoven (Netherlands), 2004).

It is a common hypothesis that the minimal performance of the char-freethermal treatment at 1000° C. as compared to the downdraft gasifier atthe same temperature suggests a catalytic role for char in tarreduction. It is possible that the nature of the fed tars (refractorytertiary tars present in fluid bed gas compared to primary or secondarytars in lower temperature pyrolysis gases) may also play a role indetermining the thermal requirement for cracking. Even so, non-catalytic(homogenous phase) tar conversion to below 200 mg tar per Nm³ of gas ispossible, starting with tar at 8000 mg/Nm³ by using ˜1150° C. for ˜4seconds. (Houben 2004). It is also notable that indirect heating onlybelow 1100° C. with short residence times (say 1075° for 2 seconds)initially increased the amount of 2+ ring polycyclicaromatics—quantifiable tars with two or more aromatic rings—but extendedresidence time mitigates this effect.

Partial Oxidation has been explored as an alternate method for achievingtar destruction. This method includes blast containing oxygensubsequently added to raw generated gas. The Energy Center of theNetherlands (ECN) performed experiments using an atmospheric circulatingfluidized bed gasifier (operated at 850° C.) where air was addedsubsequently to increase the product gas temperature to 1100° or more.(Zwart, R. W. R. Gas Cleaning, downstream of biomass gasification statusreport. Public Report, Energy Center of the Netherlands (ECN),SenterNovem, 2009.) To achieve 100 mg tar/Nm³, a temperature of 1150° C.was required, resulting in a cold gas efficiency loss of 8%.

A custom low swirl number burner (swirl number less than 0.4) wasemployed by Houben to partially combust a relatively cool (20° and 200°C.) synthetic gas feed, and so the peak temperatures were alsorelatively low (less than 900° C.). This experiment isolated (somewhat)the partial oxidation effect from thermal effect for tar destruction,and also included no effect of char. The optimum amount of blastaddition of approximately 0.2 equivalence ratio, λ, relative to the fedgas was reported to avoid growth in the PAH number (number of aromaticrings). Further, adding no oxygen with indirect heat promoted tertiarytar formation, but so did adding too much oxygen, for example λ>0.4, inPartial Oxidation.

The presence of hydrogen also seems to play a key role in tardestruction. A PAH “cracking” scheme described in Jess, A. “Mechanismsand kinetics of thermal reactions of aromatic hydrocarbons frompyrolysis of solid fuels.” Fuel 75, no. 12 (1996): 1441-1448 describesthe alternate pathways of PAH growth or PAH cracking (fewer aromaticrings and lower carbon numbers in tar compounds) that may occur withvarying hydrogen concentration. Similarly, Houben (2004) found that ifhydrogen concentration of the inlet gas were more than about 20% vol.,tar reduction was optimized. Decreasing hydrogen at the inlet below thislevel dramatically increased tar concentration in the products for thesame equivalence ratio, λ. Naphthalene and tertiary PAH (3+ ring) weretotally eliminated with an inlet hydrogen content greater than 30% vol,but single ring aromatics, e.g. toluene and benzene were retained.Therefore, gasifier operations that increase the fed hydrogenconcentration should result in beneficial tar reduction for the same POXcondition.

Catalytic tar reduction by contacting the gas with char in temperaturesin the range of 900 to 1000° C.—notably lower than necessary for thermaldestruction in the absence of char, but still elevated with respect tothe typical biomass gasifier exit temperature (750 to 850° C.)—have alsobeen disclosed. (Chen 2009). The natural minerals in biomass ash (MgO,CaO, K₂O, etc.) are believed to contribute to the catalytic effect, butthe state of prior preparation (temperature history, surface area oroxidative exposure) is also thought to impact performance. Usingcommercial biochar (active carbon) and laboratory produced biochars(using 500° C. pyrolysis) blended with sand, it was reported thatnaphthalene conversion was 99.6% and 94.4% at 900° C. with 0.3 secondsresidence time (25 cm³ catalyst bed, 2 cm bed height), starting with90,000 mg tars/Nm³ in fed gas, compared with the blank sand (2%),natural olivine and sand (55%), and dolomite and sand (61%).

Fixed carbon gasifies much more slowly (orders of magnitude more slowly)compared to the volatile matter fraction of a solid fuel at the sametemperature when under non-oxidizing conditions. A portion of theinitial fixed carbon feed is usually present as a residue ofgasification. If an appropriate technology were available to capturethis char and expose it to the flowing gas stream it could be employedas a catalyst.

Fixed char beds with direct blast addition help to achieve low tar bygaining elevated temperatures in the char bed and also by presenting thechar to the gas for possible catalytic benefit, but this approach isprone to upsets such as high temperature excursions. On the other hand,by separating the partial oxidation zone to a location above the charbed, the hot gases can be exposed to the active catalytic properties ofchar without blast input to the delicate char bed. However, a mechanicalgrate supporting low density char dust requires low superficialvelocities and this is also prone to its own possible solids flow upsets(bridging, chanelling (“rat holing”), local hot spots, etc.) due to thechaotic flow behavior of low density solids.

A partial oxidation zone that achieves higher temperatures (1150° C.)can be used to help effectively convert tars with or without passingthrough a fixed bed of char. The presence of hydrogen in the fed gas isan important feature to achieve maximum POX performance where openingaromatic rings can be favored over PAH growth. It is thought that thetype of tars produced may also impact their ability to be subsequentlyreformed on a bed of char, and may impact the cracking performance inthe POX stage; however, this has remained an unproven possibility.

The combination of these theories and principles (hydrogen rich gasproduction, followed by partial oxidation, exposure to the catalyticproperties of char, and supported char bed) in a robust and scalableindustrial design is not presently known to the state of the art. Theclassic fixed bed downdraft gasifier (and other techniques that employ afixed bed of char alone) is not scalable over about 10 MW_(th) due tothe anisotropic shape and chaotic flow potentials in low density charbeds. Blast addition directly into the fixed bed of char would not besufficiently robust for commercial deployment and would not be scalableto industrial capacities (>10 MW_(th)). The fixed bed of char alonesuffers from solids flow irregularities (“bridging”) and other processupsets (“rat holes”) that occur due its low bulk density and theanisotropic nature and non-uniform particle size of the produced char.

Generally known gasifiers are of several types. The downdraft gasifierhaving an integrated fixed bed is a classic technology that is wellknown to those skilled in the art for low-tar gas production (<300mg/Nm³). Increasing superficial velocity through the downdraft gasifier,even when there is no secondary air injection into the char bed, alsoresults in an increase of peak temperatures in the char bed. Lowest taryields are observed with high peak temperatures >1000° C. (Reed 1999)Referring now to FIG. 3, separate addition of blast into the fixed bedof char is also known to improve performance—this two stage downdraft isknown to produce the lowest tars (<100 mg/Nm³). The lowest tarperformance occurs when the secondary air (45) added to the char bed(44) is at its maximum and thus primary air at its minimum. Setting thesecondary air flow such that it is slightly below the level where“smoke” puffs out the open top describes the relativity desired ofprimary and secondary air. Peak temperatures over 1000° C. were achievedin the char zone (46). The first stage of the optimally operatedtwo-stage downdraft gasifier functions as an indirectly heateddevolatilization or pyrolysis stage (43) and is likely to producesimpler “primary” and “secondary” tar compounds. These primary andsecondary tar compounds occur at relatively low temperatures (500 to700° C.) prior to encountering the hot char bed (47). The hot char bedis believed to provide high temperature thermal cracking opportunity aswell as catalytic benefit from the biomass ash minerals.

An alternative downdraft gasifier having a separated POX zone is anotherpossibility (See FIG. 4)._One of the challenges with the classicaltwo-stage, fixed bed, downdraft gasifier is that injection of blast intoa fixed bed of char can lead to difficult operational problems—slagging(temperatures well over the ash fusion point), clinkering (fused ashparticles), chanelling (“rat holing”), fuel bridging and materialdegradation in the blast input tube.

A multi-stage gasifier system (Viking II) was developed by the DanishTechnical University (DTU) between 1980 and 1990 based on the principlesof the downdraft gasifier, but separated the blast addition from thefixed bed to improve operability. The DTU gasifier incorporates aseparate low temperature pyrolysis stage (52) (500 to 600° C.) that isconfigured above a vortex flow partial oxidation section (55) operatedto achieve peak temperatures ˜1150° C.—this stage is the only zone ofdirect blast addition. This partial oxidation zone (505) is situatedabove a downdraft, dense “fixed” bed of char (57) supported on amechanical grate (58) comprised of pivoting angle iron.

The DTU design suffers from limited scale-up potential (due to the fixedbed of char (57) and indirectly heated feed auger (51)). On the otherhand, the DTU gasifier system proved to yield very low tars (<25 mg/Nm³)and produced a rich gas with ˜25% hydrogen without steam addition, and˜35% hydrogen with steam addition. The gas quality was greatly enhancedby the recuperative indirect heat stage that can include indirect drying(52). The main difficulty with the DTU system is that the gasifiersystem still relied on a fixed bed of char, which consists of low bulkdensity solids that are irregular in particle size and shape. Arelatively low superficial velocity is believed to be required forachieving a char pile without disruption on the mechanical grate (58)(previously described)—which indicates a costly scale-up for thisreaction stage. The low density bed of char exhibits chaotic solids flowproperties that would be unmanageable in an industrial-scale system withcommercial reliability requirements.

Moving granular beds have also been used in prior art to present char asa catalyst to produced gas but in a cross flow moving granular bedfilter, see FIG. 1 (Van der Drift 2005). The Van der Drift articledescribes a laboratory experiment for validating the theory that charcan perform as a tar reduction catalyst and only employed a slip streamfrom a larger gasifier. The cross-flow design was reported to achievehigh filtration efficiency and about 75% reduction in tar at 900° C.,being presented with gas from a fluid bed that was operated at 850° C.The media retention screen (18) at the dust laden gas inlet (16) of themoving granular bed of char was reported to have dust fouling problems,and the system also suffered from media agglomeration within the gasmedia retention screens (18) which made media movement difficult.

Another moving granular bed filter which is shown in FIG. 2 is disclosedin U.S. Pat. No. 7,309,384 to Brown et al., and does not provide fordisengagement screen scrubbing nor does it provide the opportunity forextended gas residence time in the presence of char. Instead, the gasresidence time is shorter due to the rapid char and media disengagementin the counter flow design, where gas from the gas inlet (33) isadmitted at the bottom of the media bed (35) and then travels upwardfollowing path (34) mostly through a bed of clean media which wasadmitted through the media inlet (31). Extended gas-char contact thatwould otherwise benefit catalytic tar reduction by char is therefore notprovided. Further, although not shown in the patent drawing, the claimedsystem requires a gas barrier (downcomer) (39) for operation. The gasbarrier is needed to retain the media in a column and to direct the gasflow upward through the downward moving media column.

The present invention differs from the above referenced inventions andothers similar in that these prior devices do not provide features thatcan be readily scaled up to industrial operational levels. What wasneeded was a gasifier system able to meet the low tar requirements whileproducing high quality gases, and which is feasible and operable in anindustrial setting.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used in isolation as an aid in determining the scope of the claimedsubject matter. At a high level, embodiments of the invention relate toa gasification system for converting feedstocks such as biomass andwaste to combustible gases with low tar levels.

Embodiments of the invention include a gasifier wherein the char bed canbe scaled up while managing the low bulk density solids and theirregularities in operation caused by variations in superficialvelocity. Some embodiments of the invention include a gasifier thatprovides a partial oxidation zone(s) to allow maximum advantage of thehigh temperatures required for lowest tar production. Additionally,embodiments of the invention provide a gasifier that fosters highquality gas production. Further embodiments of the invention include agasifier constructed to provide disengagement screen scrubbing.

The utility of the present invention is to convert biomass and wastefeedstock (solids) into a combustible gas at elevated temperature andpressure with substantially reduced tar concentrations. There may beapplicability of this invention to gasification of other higher volatilematter solid fuels, including for example, various low rank coals, browncoal, peat, and lignite. Achieving low tar gas is the key to unlockingquantitative gas conditioning needed for advanced high efficiencygas-to-power systems (engines, combustion turbines, solid oxide fuelcells, etc.) and advanced synthesis technology for biofuels (ethanol,mixed alcohols, and Fischer-Tropsch liquids) and chemicals such ashydrogen and ammonia.

Embodiments of the invention utilize an entrained flow reactor coupleddownstream of a fluidized bed reactor. An entrained flow reactor is areactor in which the reactant feedstock and oxidant are fed into the topof the reactor so that the oxidant stream surrounds (e.g., “entrains”)the feedstock and carries the feedstock through the reactor. A fluidizedbed reactor is one in which a fluid is forced upward through a granularbed at velocities sufficient to cause the granular material to behave,in many respects, as a fluid. In certain embodiments, the entrained flowreactor incorporates a moving granular bed that captures and supports acatalytic char bed.

A first illustrative embodiment of the present invention relates to amulti-stage reaction system for producing low-tar combustible gas. Incertain embodiments, the system includes a fluidized bed reactor thatincludes a partial oxidation zone, in which a portion of the solidfeedstock is partially oxidized, thereby creating a gas and a pluralityof char particles. The illustrative embodiment further includes anentrained flow partial oxidation reactor situated downstream from thefluidized bed reactor, and where the entrained flow partial oxidationreactor includes a moving granular bed.

A second illustrative embodiment of the present invention relates to amulti-stage reaction system for producing low-tar combustible gas. Incertain embodiments, the system includes a fluidized bed reactor thatincludes a partial oxidation zone, in which a portion of the feedstockis partially oxidized thereby creating a gas and a plurality of charparticles. The illustrative embodiment further includes an entrainedflow partial oxidation reactor situated downstream from the fluidizedbed reactor. The entrained flow partial oxidation reactor includes amoving granular bed. In certain embodiments, a media screening devicescreens media from the moving granular bed and a media recycle systemreturns the screened media to the entrained flow partial oxidationreactor.

A third illustrative embodiment of the present invention relates to amethod for controlling an operating pressure of a two-stage gasificationsystem. In certain embodiments, the method includes performing a partialoxidation of a portion of the feedstock in a fluidized bed reactor;elutriating the resulting plurality of char particles and the gas fromthe fluidized bed reactor as a mixture of gas and char; receiving themixture into an entrained flow reactor that includes a moving granularbed of filtering media; and allowing the mixture to flow through themoving granular bed of char and media. As the mixture is pushed throughthe granular bed, embodiments of the method further include capturing aportion of the plurality of char particles in the filtering media;screening a portion of the filtering media to remove captured charparticles; and returning the screened filtering media to the entrainedflow reactor.

These and other aspects of the invention will become apparent to one ofordinary skill in the art upon a reading of the following description,drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in detail below with reference to theattached drawing figures, wherein:

FIG. 1 is a schematic drawing of tar reduction by tar reductionequipment in accordance with the prior art;

FIG. 2 is a schematic drawing of a counter-flow moving granular bedfilter in accordance with the prior art;

FIG. 3 is a schematic drawing of a classical two-stage downdraftgasifier in accordance with the prior art;

FIG. 4 is a schematic drawing of a downdraft gasifier incorporating anindirect heat stage and separation of the partial oxidation zone andchar fixed bed in accordance with the prior art;

FIG. 5 is a schematic drawing of a gasifier system in accordance with afirst embodiment of the invention described herein;

FIG. 6 is a schematic drawing of a gasifier system in accordance with asecond embodiment of the invention described herein;

FIG. 7 is a schematic drawing of a gasifier system in accordance with athird embodiment of the invention described herein;

FIG. 8 is a schematic drawing of a gasifier system in accordance with afourth embodiment of the invention described herein;

FIGS. 9A and 9B are top-plan diagrammatic views of nozzle placement andair flow;

FIG. 10A is a schematic drawing of a front view of a screen;

FIG. 10B is a schematic drawing of an end view of a screen

FIG. 11 is a flow diagram depicting an illustrative method of thetar-reducing gasifier system in accordance with certain embodiments ofthe invention described herein;

FIG. 12 is a chart depicting tar removal percentage vs. equivalenceratios; and

FIG. 13 is a chart showing heating value vs. total equivalence value.

DETAILED DESCRIPTION

The subject matter of embodiments of the invention disclosed herein isdescribed with the specificity required to meet statutory requirements.However, the description itself is not intended to limit the scope ofclaims in this patent. Rather, the inventors have contemplated that theclaimed subject matter might also be embodied in other ways, to includedifferent features or steps, or combinations of features or steps,similar to the ones described in this document, in conjunction withother technologies. Moreover, although the term “step” is used herein toconnote different elements of methods employed, the term should not beinterpreted as implying any particular order among or between varioussteps herein disclosed unless and except when the order of individualsteps is explicitly described.

Referring to the drawings, and particularly to FIG. 5, there is depictedan illustrative gasification system 100. The gasification system 100includes a first reactor 101 and, situated downstream from the firstreactor 101, a second reactor 102. As shown in FIG. 5, the gasificationsystem 100 also includes a media screening device 103, a media recyclesystem 104, and a heat recovery device 105. It should be understood thatthe illustrative gasification system 100 is merely one example of asuitable gasification system and is not intended to express or suggestany particular limitations regarding implementations of aspects ofembodiments of the invention.

For example, in some embodiments, the gasification system 100 caninclude any number of additional components such as, for example, thoseillustrated in FIGS. 6-8. In some embodiments, one or more of thecomponents described herein can be integrated with one another and inother embodiments, one or more of the components described herein can beseparated into any number of desired features, functions, and the like.According to various embodiments, for example, the first reactor 101 isa fluidized-bed reactor and the second reactor 102 is an entrained flowreactor. In some embodiments, the second reactor 102 can also includefluidized-bed technology, and in other embodiments, the first reactor101 can include entrained flow technology. All of these variousembodiments and implementation are considered to be within the ambit ofthe invention.

With continued reference to FIG. 5, the first reactor 101 includes anupper portion 106 and a lower portion 108. The upper portion 106 of thefirst reactor 101 includes a freeboard 110, which provides a partialoxidation zone 112. As shown in FIG. 5, the lower portion 108 of thefirst reactor 101 includes a fluidized bed 114 and a port 116 used foradding heat. The first reactor 101 also includes, as illustrated in FIG.5, a number of blast inlets 118, a solid fuel port 120, and ablast/steam inlet 122. A fluidized-bed media discharge port 124 issituated at the bottom of the lower portion 108 of the first reactor101. The fluidized-bed media discharge port 124 discharges media into amedia discharge system 125, which can carry the discharged media to anynumber of various destinations such as, for example, a waste receptacle,a storage tank, a recycling system, and the like.

In operation, the first reactor 101 creates a hydrogen-rich partialoxidation zone 112 in its upper section 106 and preferably includesdirect blast addition and/or indirect heat addition through the port116. Embodiments of the invention allow for influence and control of thehydrogen concentration in the raw gas, thereby facilitating thesubsequent cracking of tars, which occurs in the partial oxidation zone112 of the first reactor 101 and/or in a partial oxidation zone 126 ofthe second reactor 102.

With continued reference to FIG. 5, the second reactor 102 is atwo-stage entrained flow gasifier that is operated in a non-slaggingmode. The first stage is a partial oxidation stage and is accomplishedin the partial oxidation zone 126 of the second reactor 102. As shown inFIG. 5, the partial oxidation zone 126 of the second reactor 102 issituated within an upper portion 128 of the second reactor 102. Asillustrated, the partial oxidation zone 126 includes a low-swirl partialoxidation burner 136. A number of blast inlets 138 and 140 are locatedin the upper portion 128 of the second reactor 102 and will preferablyinclude a multiple of blast nozzles 142 at each level, as needed toachieve localized “thermally intense zones,” which will be described inmore detail below, with reference to FIGS. 9A and 9B. According to someembodiments, the first reactor 101 can also include a partial oxidationburner such as the burner 136. The second stage associated with thesecond reactor 102 is a “dense-bed” stage and is accomplished in acatalytic char-reduction zone 130 that is situated within a lowerportion 132 of the second reactor 102. As shown in FIG. 5, the catalyticchar-reduction zone 130 includes a moving granular bed 134 thatfacilitates operation of the second stage associated with the secondreactor 102.

Embodiments of the invention can include a number of different optionsfor configuring blast nozzles 142 around the periphery the first reactor101 and/or the second reactor 102. The configuration of the blastnozzles 142 facilitates forming localized regions of oxidative thermalintensity (by virtue of the mixing pattern), rather than achieving moreuniform mixing patterns achieved by typical approaches to designing gasburners for lean fuel conditions. Turning briefly to FIGS. 9A and 9B,top-view schematic drawings illustrate two different illustrativeconfiguration options for placement of the blast nozzles 142,respectively. As illustrated in FIG. 9A, a vessel (e.g., reactor) 145includes a number of inlet ports 147 and 149 having blast nozzles 150and 151, respectively. The blast nozzles 150 and 151 are configured inan alternating pattern such that the blast nozzles 150 and 151 directinputs toward tangent curves 152 and 154 associated with one or moretarget circles 153 and 156, the diameter of which can be variedaccording to various embodiments of the invention.

For example, as shown in FIG. 9A, the blast nozzle 150 targets thetangent 152 of a first target circle 153, which has a first diameter 157a. Similarly, the blast nozzle 151 targets a tangent curve 154 of asecond target circle 156, which has a second diameter 157 b. As shown,the first diameter 157 a can be smaller in magnitude than the seconddiameter 157 b. In other embodiments, the first diameter 157 a can belarger in magnitude that the second diameter 157 b. The targetingdirection of each of any additional blast nozzles is configured toalternate between tangent curves of the first and second target circle153 and 156. In the embodiment depicted in FIG. 9A, the blast nozzles150 and 151 are oriented at the same elevation as one another, andtherefore provide a coherent flow direction. Other flow patterns can beachieved, in other embodiments, by injection in a contrary flowdirection at slightly different elevations.

Turning to FIG. 9B, an alternative configuration option for configuringblast nozzles to achieve desirable flow patterns in a blast zone 162situated within a reactor vessel 160 is depicted. As shown, the vessel(e.g., reactor) 160 includes a number of inlet ports 163 and 165 havingblast nozzles 166 and 168, respectively. The blast nozzles 166 and 168are configured in an alternating pattern such that the blast nozzles 166and 168 direct inputs toward tangent curves of target circles that aredefined at different elevations. For example, the blast nozzle 166targets the tangent curve 171 of a first target circle 169, which has afirst diameter 173 a. Similarly, the blast nozzle 168 targets a tangentcurve 174 of a second target circle 170, which has a second diameter 173b.

In the embodiment illustrated in FIG. 9B, the first target circle 169and the second target circle 170 are situated at different elevationswith respect to one another. That is, in certain embodiments, the firsttarget circle 169 can be situated at a lower elevation than the secondtarget circle 170, while in other embodiments, the second target circle170 can be situated at a lower elevation than the first target circle169. According to various embodiments of the invention, the firstdiameter 173 a can be smaller in magnitude than the second diameter 173b. In other embodiments, the first diameter 173 a can be larger inmagnitude that the second diameter 173 b. In further embodiments, thefirst diameter 173 a and the second diameter 173 b can be substantiallythe same. The targeting direction of each of any additional blastnozzles is configured to alternate between tangent curves of the firstand second target circle 169 and 170.

According to certain embodiments of the invention, one or more auxiliaryblast zones will include multiple nozzles configured around theperimeter of the vessel so as to create at least two tangent targetcircles. In some embodiments, as depicted in FIG. 9A, the nozzles can beconfigured such that the inputs are injected coherent at the sameelevation, while in other embodiments, the nozzles can be configuredsuch that the inputs are injected convergent at slightly differentelevations. These embodiments provide differing thermal intensitypatterns and it should be understood that the configuration used inimplementation can be selected to achieve the desired thermal intensitypatterns.

Moreover, according to embodiments of the invention, mixing performanceassociated with the blast zones 146 and 162 can be optimized throughcomputational fluid dynamics (CFD) calculations. For example, CFDsoftware can be used to create 3-D patterns with thermally intense zoneshaving various peak temperatures. In certain embodiments, mixingperformance can be optimized by varying relative diameters of the targetcircles, adjusting swirl number (e.g., utilizing a swirl number lessthan 0.4), and by optimizing the equivalence ratio of the totalauxiliary blast addition. The equivalence ratio, λ, is the blast to fuelratio, relative to the stoichiometric blast to fuel required to justburn the fed gas and char. According to embodiments of the invention,the total auxiliary blast input is less than about 25% of thestoichiometric blast-to-fuel ratio calculated relative to the fedfeedstock analysis. Additionally, in some embodiments, the totalauxiliary blast can be about 50%, or more (and even up to 100%,particularly when indirect heat is supplied during the first stage), ofthe entire blast input to the reaction system.

For example, in one embodiment, the blast nozzle configuration isdeveloped using CFD software to model at least one thermally intensezone having a peak temperature of ˜1150° C. The total auxiliary blastaddition is controlled such that it has an equivalence ratio, λ, ofapproximately 0.2 (or less) in oxygen limited partial oxidation andincorporates a majority (>50%) of the total blast addition throughauxiliary ports, configured to achieve localized zones of peaktemperature of approximately 1150° C. Configuring the blast nozzlesaccordingly can facilitate achieving desired performance objectivesduring operation.

A peak temperature of between about 1000° C. and about 1200° C.generally is sufficient for activating hydrogen molecules in the mannernecessary for cracking aromatic ring compounds and providing thenecessary termination to avoid ring polymerization. Too high atemperature in the bulk gas may cause melting of ash and slag formationthat can interfere with operation. Accordingly, the partial oxidationzones are configured to include local thermally intense zones ratherthan high bulk gas temperatures. These localized thermally intense zonesfacilitate activation of hydrogen radicals that can subsequentlyinitiate chemical reactions in the adjacent bulk gas. For example,hydrogen facilitates terminating the activated carbon atom in anaromatic ring that has been thermally cracked open, thereby providingfor tar reduction rather than tar polymerization.

Returning to FIG. 5, a gas containing elutriated char 199, formed in thefirst reactor 101, and containing various natural catalytic minerals,escapes the first reactor 101 by an elutriation mechanism 200 andtravels from the elutriation mechanism 200 through a gas conduit 202.The elutriated char 199 is delivered, via the gas conduit 202, to thesecond reactor 102 through a main gas inlet 204. The maximum size anddelivery rate of the elutriated char 199 can be controlled, to a degree,by maintaining the freeboard 110 superficial velocity through control ofthe operating pressure of the gasification system 100. The elutriatedchar particles 199 pass into the second reactor 102, either in adispersed manner, through the main gas inlet 204, as shown in FIG. 1, orin a separated and concentrated form, through an auxiliary blast port140 (e.g., see FIG. 7).

By controlling the operating pressure of the gasification system 100 fora given gas production rate, it is possible to control (to a degree) themaximum particle size and the rate of release of char 199, and themaximum particle size of the char 199, through the elutriation mechanism200 associated with the first reactor 101. The particle size of the char199 affects the catalytic performance of the char 199 for gastemperatures of less than 1000° C. In other words, a smaller particlesize tends to produce more tar reduction for the same temperature,particularly if the temperature is less than 1000° C. For example, at900° tar reduction in one study was 88% for one particle size range (1to 2 mm) and 96% for another (0.1 to 0.15 mm). Accordingly, certainembodiments of the invention incorporate a method of operating thegasification system to control the char 119 particle size andelutriation rate.

According to certain embodiments of the invention, the method includes,at least in part, maintaining a target velocity in the freeboard 110 ofthe first reactor 101 by modulating the pressure set point. It will beappreciated by individuals having skill in the relevant arts thatpressure modulation can be accomplished in a number of ways such as, forexample, modulating fuel and air inputs, modulating a downstream valveposition (e.g., downstream from a particulate removal), and the like.For instance, as illustrated in FIG. 5, the gasification system 100 caninclude one or more modulating valves 210 that can be utilized tomodulate downstream particulate removals, thereby providing some levelof control over the operating pressure of the system 100.

Additionally, pressure control can be achieved by controlling the flowof char 199 through the second reactor 102. The gas 215 engaging themoving granular bed 134 in the second reactor 102 moves in co-flowdirection with granular material 135. In certain embodiments, thegranular material is input via the main gas inlet 204 of the secondreactor 102. In operation, the moving granular bed 134 captures anddilutes char 199 in a matrix of granular solids 135 that has a higherspecific gravity, thereby improving the solids' 135 flow properties. Inthis manner, a zone of gas-char 215 is created such that the gas-char215 contacts, with sufficient residence time, the char 199 solids forcatalytic tar-reduction-by-char. The moving granular bed 134 capturesand mixes the low density char 199 (usually <190 kg/m³) with other media(usually >1900 kg/m³), thereby improving the char 199 flow properties.In this manner, the flow of char 199 can be positively managed by itsassociation with the co-flowing media matrix 134.

With continued reference to FIG. 5, the concentration of char 199 in thegranular bed 134 can be managed through a screening stage, accomplishedby the media screening device 103. In one particular embodiment, forexample, a portion of char 199 and media 135 is removed, via a mediadischarge port 220 and provided to the media screening device 103. Asthe char 199 and media 135 are passed through the media screening device103, the char 199 is separated from the media 135. The media recyclesystem 104 is used to return the screened media 135 to the secondreactor 102. The char 199 screened from the media 135 is discharged viaa residue discharge system 221. According to various embodiments, theparticle size of the moving granular bed 134 can be the same as theparticle size of the fluidized bed 114 in the first reactor 101 (e.g.,see FIG. 8). In other embodiments, particle size of the moving granularbed 134 can be much larger than the particle size of the fluidized bed114 in the first reactor 101 (e.g., 10 times larger) to create afavorable pressure drop through the moving granular bed 134 in thesecond reactor 102.

Turning briefly to FIG. 11, a flow diagram depicts an illustrativemethod 300 of controlling an operating pressure of a two-stagegasification system. At a first illustrative step, step 310, theillustrative method includes performing a partial oxidation of solidfeedstock in a fluidized bed reactor. In certain embodiments, thefluidized bed reactor can be similar to, for example, the reactor 101described above with reference to FIG. 5. Performing the partialoxidation in the fluidized bed reactor generates, among other things,gas and char. The char can be elutriated from the fluidized bed reactoras a gas/char mixture, as shown at step 312, using an elutriationdevice, or simply allowed to elutriate naturally without any additionaldevice. At step 314, the mixture is received into an entrained flowreactor that has a moving granular bed. In certain embodiments, forexample, the entrained flow reactor can be similar to the second reactor102 described above with reference to FIG. 5.

As illustrated at step 316, the mixture is allowed to flow through themoving granular bed and, as the mixture moves through the movinggranular bed, char particles are captured in the media of the movinggranular bed, as indicated at step 318. To control the concentration ofchar particles in the moving granular bed (and thereby, to facilitatecontrol over the char flow rate and particle size), a portion of themedia of the moving granular bed is screened to remove char particles,as shown at step 320. At a final illustrative step, step 322, thescreened media is returned to the moving granular bed. According tocertain embodiments, the illustrative method 300 can be used alone, orin conjunction with other methods, to affect control over the operatingpressure of the gasification system by controlling the char flow rate inthe entrained flow reactor.

Returning now to FIG. 5, it should be understood that embodiments of themoving granular bed 134 do not require an inlet screen for mediaretention at the gas engagement interface by virtue of the geometricdown-flow design. In contrast, the media retention screen at thedust-laden gas inlet of the moving granular bed of the prior artillustrated in FIG. 1, for example, was reported to have dust-foulingproblems in the gas engagement. To the contrary, as a result ofemploying co-flow bed and gas and the incorporation of a down-flowdesign along with various other features of the moving granular bed 134,embodiments of the present invention do not require any media retentionscreen at the gas engagement thereby providing an improved method ofemploying char as a catalyst. Additionally, whereas conventional movinggranular beds are tuned (e.g., adjusted and controlled) to provideoptimum filtration, the moving granular bed 134 of the present inventionis tuned to provide an optimal char contacting zone.

The moving granular bed 134 is operated to capture char 199 as aphysical barrier. The media residence time is correlated with the charresidence time (the period of time that the average char particle spendsin the reactor), and this char residence time can be modulated in acontrolled manner with the media screening and recycle subsystem(103/104). According to certain embodiments, the moving granular bed 134also can be configured to provide a zone of narrow gas residence timedistribution through the char bed 134 in a conceptually plug flowreactor that allows for maximum tar cracking. In certain embodiments,the gas residence time and char residence time can differ by severalorders of magnitude; therefore, the differential velocity between thegas 215 and char 199 is very close to the local gas interstitialvelocity through the bed 134. In certain embodiments, the char in thechar bed 134 is continuously refreshed by the char 199 supply from thefirst reactor 101 thereby reducing or eliminating the need for highperformance filtration, even though some small particles of char 199 mayslip through the bed with the gas 215.

With continued reference to FIG. 5, the moving granular bed 134 includesa substantially vertical gas disengagement screen 222. Turning brieflyto FIG. 10A, the disengagement screen 222 is comprised of a plurality ofwires 224, oriented in a substantially parallel and vertical manner. Thewires 224 are situated between an upper frame edge 222 a and a lowerframe edge 222 b. As illustrated, the screen 222 can also include a pairof side frame edges 222 c. The disengagement screen 222 includes anumber of gaps 225, each gap 225 being defined between two adjacentwires 224. FIG. 10B depicts a bottom, partial view of the screen 222. Asshown in FIG. 10B, each wire 224 may be made of commercially availabletriangular profile wire, or includes a wedge or V-cross section definedby a first side 230 and a second side 232 that meet at a vertex 234. Thetwo sides 230 and 232 of the wire 224 converge (at the vertex 234) inthe direction of gas flow. Preferably, the gaps 225 between the wires224 are designed relative to the smaller cut size of the granular media.The substantially vertical orientation (which, at a minimum, is steeperthan the angle of repose of the char-media matrix 135 of FIG. 5) of thegas disengagement screen 222 provides for scrubbing action with themoving bed 134 to maintain the gas disengagement screen 222, withoutplugging.

Embodiments of the moving granular bed 134 of the present inventioninclude features that are not known to the art and that have beendescribed above. These features include, for example, a co-flow designthat is preferred for its gas-char contacting zone for enhancingcatalytic tar-reduction-by-char performance rather than for its filterperformance; the lack of a need for a media retention screen for mediaretention at the gas engagement interface; and a substantiallyvertically oriented gas disengagement screen (e.g., which is steeperthan the angle of repose of the blended char and media matrix to scrubthe disengagement screen keeping it free of dust clogging).

In contrast, for example, the prior art moving granular bed filterdisclosed in U.S. Pat. No. 7,309,384 to Brown et al., and illustratedherein in FIG. 2, does not provide for disengagement screen scrubbing.The counter filter 304, 307 disclosed in the Brown et al. patent alsodoes not provide the opportunity for extended gas residence time in thepresence of char. Instead, the gas residence time is shorter due to therapid char and media disengagement in the counter flow design which, inturn, shortens the gas-char contact and reduces the catalytic reductionof tar by char. To the contrary, embodiments of the present invention donot include, or require, the presence of a gas barrier for mediaretention whereas the Brown et al. invention will not work without sucha barrier.

According to various embodiments of the invention the residence time(increased internal age distribution) of the trapped char particles iscontrolled by modulating the media flow that captures the char throughan external recycle loop. With reference to FIG. 5, a first embodimentemploys a bubbling fluid bed reactor 101, the transport disengagementheight of the freeboard 110 design, and the target operating velocity toachieve a mixture of gas 215 and char 199 delivered to the secondreactor 102. As illustrated, the gasification system 100 of FIG. 5includes an “external” active media recycle system 104 (which can be,for example, mechanical or pneumatic conveying). In certain embodiments,the fluid bed reactor 101 can include processing of the discharge stream125 to remove foreign “tramp” materials entering with the feedstock, andcan optionally be recirculated and reheated in a direct or indirectheating loop and returned to the first reactor 101 via the port 116.

Turning to FIG. 6, another embodiment of a gasification system 240 isillustrated. As shown, the gasification system 240 includes a firstreactor 241, which is a fluidized bed reactor, a second reactor 242having a moving granular bed 243, a media screening device 244, a mediarecycle system 246, and a heat recovery device 248. This particularembodiment (illustrated in FIG. 6) is designed to enable a higher, moreturbulent, velocity in the fluidized bed reactor 241 without excessiveloss of fluid bed sand. In this case, a sand recovery roughing cyclone250 is an elutriation device included between the first reactor 241 andthe second reactor 242.

Turning now to FIG. 7, another embodiment of a gasification system 260of the present invention is illustrated. As shown, the illustrativegasification system, according to this particular embodiment, includes afirst (fluidized bed) reactor 261, a second (entrained flow) reactor 262having a moving granular bed 263, a media screening device 264, a mediarecycle system 265, and a heat recovery device 266. In this embodiment(illustrated in FIG. 7), the sand recovery cyclone 268 and turbulentfluid bed 263 are both included, but a char-concentrating cyclone 270 isalso included to give an opportunity for separate, controlled charinjection into the partial oxidation zone 276 of the second reactor 262.The embodiment, illustrated in FIG. 7, includes the char fines cyclone270 to create an opportunity for a majority of char to bypass an upperpartial oxidation zone 277 in the second reactor 262. This configurationaffords separate control of char injection using a steam motivatedeductor 272 for delivery into the partial oxidation zone 277 through anauxiliary partial oxidation blast port 274. According to certainembodiments, the gasification system 260 can be optimized for charoxidation heat release by adding blast along with the char feed,therefore creating a hot zone through which the fed gas passes, in whichcase a lesser amount (or, in some implementations no amount) of blastgas is fed through an upper blast port 279.

Turning to FIG. 8, another embodiment of a gasification system 280 isillustrated. The illustrated embodiment of FIG. 8 is a new form of acirculating fluidized bed reactor system 280. The illustrativegasification system 280 illustrated in FIG. 8 is quite different fromthe previously described embodiments herein in several respects. Forexample, this particular embodiment (shown in FIG. 8) requires noexternal media recycle system. Instead, the same media 283 is cycledbetween the first reactor 281 and the second reactor 282, and charconcentration is modulated by adjusting the portion of discharged sand290 that either returns to the first reactor 281 or that passes throughthe media screen 284 before recycling. There is no separate mediarecycle loop for the second reactor 282 in the embodiment depicted inFIG. 8. Circulated media 292 is the same particle size and same materialused in the fluidized bed reactor 281. Bed material is circulated in aclosed loop between the fluid bed reactor 281 and the second reactor282. According to embodiments, the bed can have a ratio of superficialvelocity relative to the minimum velocity required to fluidize (U/Umf)of between 6 and 12 to cause increased sand elutriation. Dust-ladenmedia discharging flow (indicated as 290) is split by gravity assistand/or pneumatic push to return a portion into the fluid bed 281 a andthe balance is processed to remove fines and dust by the media screeningdevice 284 as needed, to produce a cleaned media stream 296. The mediastream from the fluid bed 281 a and the media screening device 284combined 298 can optionally be used in a direct or indirect heating loopor cleaned of tramp materials and returned via the port 281 b.

To recapitulate, embodiments of the invention include a gasificationsystem having a fluidized bed reactor situated upstream from anentrained flow reactor. The entrained flow reactor includes a movinggranular bed that holds up char and so presents catalytic properties ofchar to the gas for the purpose of tar cracking. In some embodiments,char concentration in the granular solids matrix is controlled with themedia screening and recycle system (such as the media screening device103 and the recycle system 104 illustrated in FIG. 5). Tests have shownthat a gasification system having at least some of the featuresdescribed herein performs as desired.

For example, tests were performed under various operating conditions ina laboratory-scale entrained flow reactor, configured according toembodiments of the invention, using yellow seed cord as a modelfeedstock. Air or oxygen was used as blast as indicated in FIG. 12. Thestoichiometric air/fuel requirement is 5.45 kg air/kg biomass (dry), or1.26 kg oxygen/kg biomass (dry). Air blown gasification tests used 3.7kg/hr biomass, and oxygen blown tests used 6 kg/hr biomass with ˜0.45 kgsteam/kg biomass. Inert rock material and low grade iron ore (taconite)were used as media in the entrained flow reactor, sized to approximately⅛″×¼″ granules. The first stage (fluidized bed) gasifier was operated atvarious equivalence ratios, λ, (Air/Fuel relative to the stoichiometricAir/Fuel requirement): 0.11, 0.14, and 0.18, as indicated in FIG. 12 andachieved fluid bed temperatures from 600 to 700° C.

The entrained flow reactor consisted of a small partial oxidation burnerthat injected blast laterally through 6 small holes (having an internaldiameter of 1.4 mm) with slight swirling action, in the configurationillustrated in FIG. 9A, with a tangent circle that is 13 mm diameter,inside of a 40 mm inside diameter pipe, that subsequently expanded intoan 100 mm (inside diameter) pipe. Test results indicate that tars wereconverted with increasing blast (and increasing temperatures) up to apoint, which correlated with delivering ˜50% of the total blast into thesecondary reactor. The lowest tar condition was ˜560 mg/Nm³ dry gas. Thetemperature in the partial oxidation zone was not measured, but themaximum measured temperature in the zone below was 980° C.

As another example, further tests were performed in which the same testconditions discussed above were analyzed for lower heatingvalue—indicate that the heating value stabilized at ˜5 MJ/Nm³ (dry) forair blown tests, and 10 MJ/Nm³ (dry) for oxygen blown tests, so long asthe total equivalence ratio was less than 0.35, as reflected in FIG. 13.

A reactor, configured in accordance with embodiments of the invention,that combines a partial oxidation burner for tar reduction above a“pebble grate” (which is previously described as a bed of granular mediasupported by a vertical wire screen supporting the media at the gasdisengagement) to support a bed of char has not heretofore been known tothe art. The integration of a first reactor that operates at lowertemperatures in sequence with a higher temperature partial oxidationstage (sub-stoichiometric combustion) with a subsequent heatrecuperative device that transports thermal energy back to the firstreactor to drive pyrolysis reactions is also not previously known.Because most moving granular beds are designed for filtration, theco-flow design of the present invention is also not known because itdoes not impart ideal filtration conditions but rather it imparts idealgas-char contact conditions for tar cracking. Low density char—thatotherwise has chaotic solids flow properties—is dispersed into agranular bed material that has approximately ten (10) times the bulkdensity, which imparts improved solids flow properties. This is animprovement over prior art and contrasts with any reactor that includesa fixed bed of char in downdraft type gasifiers (integrated or separatedfrom partial oxidation zones) that have been known to experience upsetsassociated with poor solids flow, “solids bridging”, and “rat hole”formations due to the chaotic movement and anisotropic nature of lowdensity char beds.

The present invention has been described in relation to particularembodiments, which are intended in all respects to be illustrativerather than restrictive. Alternative embodiments will become apparent tothose of ordinary skill in the art to which the present inventionpertains without departing from its scope. For example, differentcontact times and variations in temperatures for certain zones may beemployed. Connections between reactor vessels and return lines can varyin general position. It will be appreciated by individuals having skillin the relevant arts that certain optimizations will be necessarydepending on the source of feedstock.

From the foregoing, it will be seen that this invention is one welladapted to attain all the ends and objects set forth above, togetherwith other advantages, which are obvious and inherent to the system andmethod. It will be understood that certain features and subcombinationsare of utility and may be employed without reference to other featuresand subcombinations. This is contemplated by and is within the scope ofthe claims.

1. A multi-stage reaction system for producing low-tar combustible gas,the system comprising: a fluidized bed reactor that includes a partialoxidation zone, in which a gas and a plurality of char particles arecreated in said partial oxidation zone; and an entrained flow partialoxidation reactor positioned downstream from the fluidized bed reactor.2. The system disclosed in claim 1 wherein said entrained flow partialoxidation reactor includes a moving granular bed.
 3. The system of claim1, wherein the fluidized bed reactor further comprises a freeboard, saidfreeboard operated at a velocity controlled to create a generallyconsistent char particle size feed.
 4. The system of claim 2 whereinsaid plurality of char particles is of generally consistent charparticle size and at least a portion of said plurality of char particlesis provided to the entrained flow partial oxidation reactor.
 5. Thesystem of claim 3 wherein provision of said generally consistent charparticle size comprises modulating a pressure of the fluidized bedreactor.
 6. The system of claim 1, further comprising a sand recoverycyclone.
 7. The system of claim 1, wherein the entrained-flow partialoxidation reactor further includes a partial oxidation zone and meansfor injecting a stream of combined gas and char into said partialoxidation zone.
 8. The system of claim 7, further comprising a cyclonefor concentrating the plurality of char particles out of a fed gasstream prior to injection into said partial oxidation zone.
 9. Thesystem of claim 8, wherein said cyclone concentrates the plurality ofchar particles out of the fed gas stream prior to injection into saidpartial oxidation zone.
 10. The system of claim 2, wherein theentrained-flow partial oxidation reactor includes a plurality of blastinlet ports configured around a periphery of a vessel enclosing thereactor.
 11. The system of claim 10, wherein said plurality of blastinlet ports is arranged in an alternating pattern such that each of theplurality of blast inlet ports targets a tangent curve of a tangentcircle.
 12. The system of claim 10, wherein said plurality of blastinlet ports is arranged in an alternating pattern such that a first oneof the plurality of blast inlet ports targets a first tangent curve of afirst tangent circle and a second one of the plurality of blast inletports targets a second tangent curve of a second tangent circle, whereinthe first and second tangent circles are located at the same elevation.13. The system of claim 10, wherein said plurality of blast inlet portsis arranged in an alternating pattern such that a first one of theplurality of blast inlet ports targets a first tangent curve of a firsttangent circle and a second one of the plurality of blast inlet portstargets a second tangent curve of a second tangent circle, wherein thefirst and second tangent circles are located at different elevations.14. The system of claim 7, wherein the moving granular bed operates inco-flow with respect to the stream of combined gas and char to create agas-char contacting zone.
 15. The system of claim 14, wherein agas-media disengagement screen is oriented at an angle that is steeperthan an angle of repose of the combined media and char mixture.
 16. Thesystem of claim 15, wherein the gas-media disengagement screen isoriented substantially vertically.
 17. The system of claim 15, whereinthe gas-media disengagement screen includes a plurality of parallelwires extending between an upper frame edge and a lower frame edge. 18.The system of claim 17, wherein each of the plurality of wires includesa cross section partially defined by a first side that converges with asecond side at a vertex in the direction of the disengaging gas flow.19. The system of claim 18, wherein the cross section is wedge shaped.20. The system of claim 2 wherein the entrained-flow partial oxidationreactor further includes a partial oxidation zone, means for injecting astream of combined gas and char into said partial oxidation zone, and aplurality of blast inlet ports configured around a periphery of a vesselenclosing the reactor.
 21. The system of claim 20 wherein the movinggranular bed operates in co-flow with respect to the stream of combinedgas and char to create a gas-char contacting zone.
 22. The system ofclaim 2 wherein the entrained-flow partial oxidation reactor furtherincludes a partial oxidation zone and means for injecting a stream ofcombined gas and char into said partial oxidation zone and said movinggranular bed includes a gas-media disengagement screen oriented at anangle steeper than the angle of repose of the combined media andplurality of char particles.
 23. A method for controlling an operatingpressure of a two-stage gasification system, the method comprising:performing a partial oxidation of a portion of biomass in a fluidizedbed reactor, wherein the partial oxidation creates a gas and a pluralityof char particles; elutriating at least a portion of said plurality ofchar particles and gas from the fluidized bed reactor, wherein saidelutriating includes removing a mixture of gas and char particles fromthe fluidized bed reactor; receiving the mixture into an entrained flowreactor, wherein the entrained flow reactor includes a moving granularbed of filtering media; allowing the mixture to flow through the movinggranular bed; and capturing a portion of the plurality of char particlesin the filtering media.
 24. The method of claim 23 further comprisingscreening a portion of the filtering media to remove captured charparticles; and returning the screened filtering media to the entrainedflow reactor.
 25. A multi-stage reaction system for producing low-tarcombustible gas, the system comprising: a fluidized bed reactor thatincludes a partial oxidation zone in which a portion of the feedstock ispartially oxidized, wherein said partial oxidation creates a gas and aplurality of char particles; an entrained flow partial oxidation reactorsituated downstream from the fluidized bed reactor, the entrained flowpartial oxidation reactor including a moving granular bed; a mediascreening device that screens media from the moving granular bed; and amedia recycle system that returns the screened media to the entrainedflow partial oxidation reactor.